Heart transplant links donor, recipient with different blood types

Pediatric cardiac surgeons at the Monroe Carell Jr. Children’s Hospital at Vanderbilt have performed the state’s first ABO incompatible heart transplant in a 2-month-old patient.

The procedure — reserved for children 2 years old and younger who have not yet developed significant levels of anti-ABO antibodies — helps a transplant recipient tolerate an organ from a donor whose blood type is incompatible.

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When people who have one blood type receive blood from someone with a different blood type, it may cause their immune system to react. This is called ABO incompatibility.

These types of transplants are rare, mostly due to current donor and recipient matching algorithms. But pending changes to the United Network for Organ Sharing (UNOS) protocol give physicians at Vanderbilt hope that the practice will become standard treatment.

“We are the only center in the state doing ABO incompatible heart transplants,” said Debra Dodd, M.D., medical director of the Pediatric Heart Transplant Program at the Monroe Carell Jr. Children’s Hospital at Vanderbilt. “Not every transplant center in the country will be able to perform this procedure. Our patients listed as ABO incompatible will now have access to organs that previously were not available to them. They will have an improved chance, rather than dying while waiting.”

In 2006 UNOS agreed to allow incompatible organ listing for heart transplant patients 6 months old and younger. A year later regulations changed to include candidates 2 years old and younger.

Debra Dodd, M.D., and Bret Mettler, M.D. (photo by Steve Green)

Now discussions are underway to permit compatible and incompatible heart transplants to be listed equally.

“We are hoping it will happen in the coming year,” she said. “Making this change will further shorten the waitlist time for this patient population as well as improve long-term survival. It will significantly impact the children who come to us for hearts.”

In the past, children listed as ABO incompatible for heart transplants were at the “bottom of the list” in terms of matching. But recent outcomes showing improved survival rates have sparked growing interest in the technique.

Once UNOS’ complex, computerized matching system finds a recipient for an organ, the medical team has a couple of hours to prepare the recipient for transplantation. But for incompatible organs, there are a few extra steps required for a successful outcome.

“This procedure, which is done in the Operating Room, allows us to modulate the immune system of our patient to allow them to tolerate a transplant from a donor whose blood type is incompatible,” said Bret Mettler, M.D., assistant professor of Pediatric Cardiac Surgery.

“At the time of their operation, we do an exchange transfusion — which means we replace all of their native blood volume with blood from our blood bank, minimizing the immunologic response to the transplanted heart causing hyperacute rejection.”

Mettler said the medical team checks the antibodies in the blood prior to re-perfusing the organ once transplantation is complete to ensure that the levels are low enough to prevent rejection. If the levels are too high, another exchange is done.

“With an exchange transfusion and an immature patient’s immune system, as they age, they will develop tolerance to the organ that is from a different blood type,” he said. “The long-term outcomes are equivalent to patients who have received ABO compatible transplantation.”

For nearly two decades, Canada and some European countries have been performing pediatric ABO incompatible heart transplants. Dodd said their high mortality rates while waiting for a donor prompted them to seek alternative treatment options.

Since pioneering the incompatible transplant, Canada has seen a dramatic decrease in transplant mortality rates — from 58 percent down to 7 percent.

Dodd anticipates similar improvements for both Vanderbilt and other eligible centers nationwide as the practice becomes standard.

The pending UNOS change in criteria could also result in a 20 percent increase in heart transplant volume at Vanderbilt, which is one of a handful of centers in the Southeast that perform ABO incompatible heart transplants. Fewer than 20 facilities offer the procedure in the U.S.

“This is a testament to multiple disciplines collaborating within Vanderbilt,” said Mettler. “To be able to perform an equally complex medical and surgical procedure in the most fragile population very early in life is very satisfying. There are many areas within the hospital that come together to make this work.

“We hope that being able to offer ABO incompatible transplants leads to fewer children dying on the waiting list waiting for an acceptable organ,” Mettler said.